Why can't Canon make a smaller, lighter, less-expensive 500 f/4 or even a 600 f/4 for my 7D? I think if Canon did that, or Nikon, maybe even both, then the appeal of FF cameras would go way down because of the tremendous appeal of lighter and less-expensive lenses for the APS-C cameras. But almost all the good lenses--not all, please don't jump on me--are designed for FF. To name a few from Canon that have no APS-C equivalent: 300 f/2.8 and f/4L; 70-200 f/2.8 and f/4L; 400 f/5.6L; 100-400 f/4.5-5.6L; all the super-teles, all the macros; and so on and on.

There would be no point. Look at the 17-55/2.8IS, an excellent lens, designed only for APS-C. For all practical purposes, it is nearly as big and heavy as the 24-70/2.8.

Exacerbating this problem is that APS-C SLRs still have a FF flange distance, so the lens is further from the sensor than it needs to be, so I imagine that also makes them bigger. And that is never going to change for Canon or Nikon. I suppose some other company (Pentax?) could bid farewell to the idea of ever going FF and invent a new mount and system that was only APS-C, with a shorter flange distance, but I don't see that happening. The market would be pretty small - people who really want both a smaller camera AND an optical TTL viewfinder.